Polyalkylene terephthalate represented by polyethylene terephthalate (hereafter referred to as PET for short) is produced by reacting a lower alcohol ester derivative of terephthalic acid or a dicarboxylic acid containing terephthalic acid as a major component with a glycol so as to generate a glycol ester of terephthalic acid and/or an oligomer thereof, and subjecting the oligomer to a polycondensation reaction under high vacuum. In order to further increase the molecular weight, a method in which the obtained polymer is further solid state polymerized in a solid state under a high vacuum or in an inert gas, is also industrially employed.
As a polycondensation catalyst of a PET, antimony catalysts represented by an antimony trioxide have been used for a long time. However, an alternative for the antimony catalysts is desirable from the standpoint of environmental load. A PET using an antimony catalyst has a quality problem in that it is difficult to dye clearly, because a part of antimony becomes antimony black and the polymer becomes dull black. Further, during spinning, antimony black causes contamination around a spinneret and filter clogging, and as a result thread breakage may occur. It is necessary to clean the spinneret and the filter to get rid of these problems, and there has been a productivity problem in that a cleaning cycle becomes short.
In such a situation, there have been some studies of alternative materials of antimony catalysts from a standpoint of environmental issues and improving quality and productivity. In particular, a lot of studies using organic and inorganic compounds including titanium have been conducted.
Patent Document 1 discloses a preferable transparent titanium oxide sol and also discloses that titanium oxide is a fine particle of about 400 to 500 mμ and that optical transparency of titanium oxide sol at a wavelength of 370 to 600 nm is not less than 95%. However, the point where a specific amount of hydroxyl groups is included and the point where there is an effect as a polymerization catalyst for a polyalkylene terephthalate are not disclosed. Further, according to a study by the present inventors, the hydroxyl groups in the titanium oxide disclosed here are less than 1.8 mmol per 1 g of titanium oxide. This is because the production method is different from that of the titanium oxide of the present invention and the titanium oxide particles are simply pulverized or produced by a vapor phase method.
Patent Document 2 discloses a technique using a powdered titanium oxide larger than 10 m2/g in a specific surface area as a polymerization catalyst of polyester, but does not describe that using a specific titanium oxide sol is effective for a polymerization catalyst. These inorganic titanium compound catalysts are considered better catalysts because their thermal decomposition reactivity are low although they still have higher activity compared with organic titanium compounds. However, there is still a case where the obtained polymer may become stained severely, and to avoid this, using a cobalt compound, which is a bluing agent, has been considered. However, like antimony compounds, cobalt compounds are not necessarily effective for resolving environmental problems and further, and as for the quality of a polymer, there has been a tendency to decrease thermal stability of polymers and clarity of dye materials.
Patent Document 1: JP 10-182152 A
Patent Document 2: JP 2000-119383 A